1 [Field of the Invention]
The present invention relates to a heat-sensitive recording material. In particular, the present invention relates to a heat-sensitive recording material characterized by containing 1,2-bis(3,4-dimethylphenyl)ethane as sensitizer.
2 [Description of the Prior Art]
Heat-sensitive recording materials consist of a heat-sensitive color-forming layer comprising a dispersion of a sensitizer, binder and other additives in a coupling system comprising a usually colorless or light-colored coupling substance such as a leuco dye and a developer which causes coloring of the coupling substance upon heating, said layer being formed on a support such as paper, synthetic paper or resin film. When a heating element such as a thermal head or hot pen is brought into contact with the recording material in a recording device, the dye is reacted with the developer to develop a color such as black to thereby form a record.
The heat-sensitive recording materials are widely used in instrumental recorders, computers, facsimiles, telex devices, automatic passenger ticket vending machines, etc., since they are superior to other recording materials in that the records can be obtained in a short time, the noise is only slight and they are inexpensive.
As the colorless or light-colored coupling substances, for example, leuco dyes having a lactone, lactam or spiropyran ring are used. As the developers, various acidic substances have been proposed heretofore. Among them, phenolic compounds such as bisphenol A and benzyl p-hydroxybenzoate are frequently used either alone or in combination. However, when these phenols are used, increase of the recording speed and density is difficult, color shading is caused or discoloration occurs during the storage disadvantageously.
Under these circumstances, it was proposed to add a sensitizer to the coupling substance and developer to increase the sensitivity. As the sensitizers, waxes, dimethyl phthalate, stearamide, phenyl benzoate, terphenyl, bis(vinyloxyethoxy)-benzene and p-acetyloxybiphenyl were proposed.
However, these sensitizers are practically unsatisfactory, since they have defects that the effects of them cannot be sufficiently obtained unless they are used in a large amount, a high sensitization of the heat-sensitive recording material required thereof cannot be sufficiently satisfied, fogging is caused and discoloration occurs during the storage.
Japanese Patent Publication No. 25674/1984 discloses the use of an alkylated biphenyl or substituted biphenylalkane having a melting point of 60.degree. to 200.degree. C. as the sensitizer. It is also described therein that when such a sensitizer is used, a coupling sensitivity higher than that obtained when stearamide is used can be obtained. Suitable examples of the substituted biphenylalkanes described in the Publication are 1,2-bis(2,4-dimethylphenyl)ethane and 1,2-bis(2,4,5-trimethylphenyl)ethane.
However, even when such a compound is used, the effect is yet insufficient and practically unsatisfactory.
For example, although the coupling sensitivity is improved to some extent when 1,2-bis(2,4-dimethylphenyl)ethane is used, the storability is poor, fog is formed in the non-image area and the colored part is seriously discolored. When 1,2-bis(2,4,5-trimethylphenyl)ethane is used, the coupling sensitivity is quite low and the discoloration in the colored area is disadvantageously severe, though the fogging in the non-image area is slight.